This invention relates to a process for producing improved high temperature noise attenuation panels and more particularly to sandwich type attenuation panels utilized in a severe environment including elevated temperatures as high as 1600.degree. F. An example of its use is for sound attenuation within the exhaust area of a modern jet aircraft engine.
In the design and manufacture of sound attenuation sandwich panels which additionally provide structural integrity in severe environments, it has been common practice to provide an attentuation sandwich panel wherein honeycomb core material is sandwiched between a perforate and imperforate sheet of thin facing material. Panels of this type of construction, although satisfactory for attenuating some specific sound frequencies, are found to be inadequate for a broad range of frequencies customarily encountered within and around modern jet engines. It has also been found that the perforations, when placed adjacent to the high speed gas and in airflow areas within aft engines, create some turbulence in those high speed flows. It has been further discovered that these panels are generally constructed with an adhesive which limits their use to low temperature environment to prevent their degradation or failure. Although a co-pending application by the assignee of the instant invention having Ser. No. 604,200 reduces and substantially eliminates this turbulence problem and the narrow frequency problem for attenuation sandwich panels in or around modern jet engines, this new concept attenuation sandwich panel cannot be used in elevated temperatures due to its adhesive bonded construction.
It should be appreciated that the environment in an aero-engine duct is very severe in that there exists a very high sound pressure level, duct dimension variations, temperature variations, boundary layer thickness variations, and a varying velocity of air flowing along the duct liner surface commonly called the grazing flow. This environment is especially severe in the exhaust area of an aero-engine where extremely high operating temperatures are encountered. The term acoustic liners is also meant to include such nacelle and engine structure as tail pipes, tail plugs, and tail cones and is not meant to be limiting.
Before the present invention, there had not been an entirely satisfactory attenuation material with structural integrity capable of withstanding severe high-temperature environment conditions, such as those encountered in exhaust areas of modern jet aircraft engines.